This invention relates to an apparatus and method for actuating arms, and in particular to an apparatus and method for the controlled actuation of a plurality of arms as may be used on a borehole data-logging tool such as a measuring sonde, and be used to deploy measuring instruments against a borehole wall.
1. Background of the Invention
Boreholes are drilled into the earth for the extraction of oil or gas, for example, or for the analysis of rock to determine whether oil or gas might be present. Following drilling of the borehole, a data-logging tool may be introduced into the borehole to provide data upon the borehole and the surrounding rock.
A very basic use of a data-logging tool is to determine the borehole transverse dimensions by measuring the cross-sectional dimensions of the borehole at chosen positions within the borehole. A more sophisticated data-logging application is the taking of measurements within the borehole which can indicate the location and direction of rock strata, for example.
2. Description of the Prior Art
A typical borehole data-logging tool comprises a cylindrical mandrel carrying one or more arms, these arms being mounted to pivot relative to the mandrel. By various means the arms are kept substantially parallel to and within the circumference of the mandrel while the tool is conveyed to the zone of interest in the borehole. When measurements are required the arms are rotated on their pivots so as to swing their distal ends outwards until they make contact with the borehole wall.
In basic data-logging applications the cross-sectional dimensions of the borehole can be determined from the distances to the contact points from the mandrel. By analogy with traditional hand tools used to determine the distance between two points, the arms used in this way are referred to as calipers. These distances typically are calculated from measurements of the internal movement of the opening mechanism and knowledge of the geometry of the mechanism and the arm lengths.
In elementary caliper tools, an opposed pair of arms are coupled together so as to open symmetrically about the mandrel, so that the mandrel must be centered within the borehole for both of the opposed pair of arms to contact the wall. A second pair of such arms may be arranged rotationally about the longitudinal axis of the mandrel a quarter-turn from the first pair, to give a second cross-sectional dimension. If the borehole is elliptical in cross-section then typically the tool will rotate into alignment such that the two cross-sectional dimensions are measured along the principal axes of the ellipse. However, the borehole will often be other than substantially vertical, and the weight of the tool will typically cause the mandrel to lie closer to the lower side of the borehole. Because the arms are linked in opposed pairs, the uppermost arm of at least one of the pairs may not make contact with the borehole wall.
Even if the borehole is circular. so that the cross-sectional dimensions are diameters of the borehole, unless the borehole is substantially vertical a proportion of the weight of the tool will be borne by the lowermost arm (or arms), and it is necessary for that arm (or those arms) to force the tool into a central position within the borehole so that the opposed arm(s) can contact the borehole wall.
In a more advanced tool as disclosed in U.S. Pat. No. 4,715,440, the arms are independently pivoted so that borehole irregularities can be determined and so that centralisation of the mandrel is not required. This tool uses a motorised screw mechanism in which an internal plate is translated longitudinally by rotation of the screw. The plate presses against a set of springs for each of the arms, the springs in turn causing movement of a link which can pivot a respective arm open or closed. The provision of the springs between the plate and each link allows the arms to attain independent pivoted positions relative to the mandrel.
A major disadvantage of this tool is that the speed of opening is substantially constant, so that although fine adjustment of contact force can be obtained, the time taken to move the arms from closed to open is slow, reducing the suitability of this tool to take measurements close to the bottom of the borehole.
Measuring to the bottom of a borehole is often important to maximise the knowledge obtained, and on occasion to determine if additional drilling is required. However, the fluid in the bottom of the borehole will often have been left stagnant for many days prior to measurements being made. Besides debris which might be present at the bottom of the hole, mud particles, which are deliberately introduced into the borehole so as to increase the fluid density and to prevent the borehole collapsing, will often have sunk to the bottom of the borehole during this period, rendering the fluid there relatively heavy and tenacious. It is well-known that the presence of such mud results in a high risk of the tool becoming stuck if it is allowed to dwell therein. When the tool reaches close to the bottom of the borehole, it is therefore desirable to be able to open the arms rapidly so as to be able to commence data-logging and allow subsequent retrieval of the tool within a few seconds. Such rapid opening is not possible with the tool or method disclosed in U.S. Pat. No. 4,715,440.
A known means of accomplishing rapid opening is to introduce the tool into the borehole with energy stored in a compressed spring, and to provide a means to release the spring so as to activate the arm opening mechanism with high force, and rapid opening, once the tool is in its chosen position. One means by which this may be achieved is disclosed in U.S. Pat. No. 4,594,552 which includes a single arm biased outwardly by a leaf spring. A major disadvantage of this tool is that only one arm is provided.
U.S. Pat. No. 4,056,004 discloses a tool having four arms, each of which can carry a sensor pad or other component which is desired to be moved into contact with the borehole wall. Each arm has its own spring and is biased outwardly independently of the other arms. In one embodiment each arm comprises a respective bow spring attached at each of its ends to the body of the tool; in another embodiment each arm comprises linkages which are also connected at each end to the tool, with a spring acting upon one end of the linkage to bias the center of the linkage outwardly. A restraining means is provided to hold the arms in their retracted positions, the restraining means comprising a longitudinally movable member which can act upon one of the ends of the bow springs (or linkages) to increase the distance between the ends thereof and so force the bow springs (or linkages) to lie substantially along the longitudinal axis of the tool. The restraining means described is solenoid actuated, but is indicated alternatively to be hydraulically or pneumatically actuated.
A major disadvantage of the disclosures of U.S. Pat. Nos. 4,594,552 and 4,056,004 is that there is no means to regulate the contact force between the sensor pads and the borehole wall, and the contact force will vary with the borehole size, i.e. the force imparted by the arms upon the borehole wall is dependent upon the distance by which the arm must be opened to engage the borehole wall. Also, if U.S. Pat. No. 4,056,004 is being used in an a circular borehole such as that shown in the drawings, the contact force for one of the arms may differ significantly from the contact force of another of the arms. Another major disadvantage is that the spring force is constantly acting, and any failure of the restraining means or in its control circuitry will cause the arms to move outwardly, perhaps preventing removal of the tool from the borehole.
The aim of the present invention is to reduce or avoid the disadvantages of the prior art arrangements described above.
The invention provides an apparatus for actuating arms comprising a mandrel, and at least one arm carried by the mandrel, the one more arms being mounted to the mandrel to pivot between an expanded position, in which a part of the arm projects from the mandrel, and a retracted position. The one or more arms have a resilient biasing means. A drive means is provided, adapted to load the resilient biasing means of all of the pusher means. A restraining means, comprising a hydraulic piston and cylinder assembly, is associated with each arm, a separate restraining means being provided for each of the arms, and it is arranged that release of the restraining means permits the arms to move in response to a force provided by the resilient biasing means.
The drive means can also be a hydraulic piston and cylinder assembly. Actuation of the drive means whilst the arms are in contact with the wall of the borehole can be used to increase or decrease the contact force. Thus, it will be understood that when the apparatus is in use, with all of the arms in contact with the borehole wall, each of the resilient biasing means is imparting a contact force to the arm. Actuation of the drive means can further load the resilient biasing means to increase the contact force, or can partially release the resilient biasing means to reduce the contact force. The drive means can also release the resilient biasing means, reducing the force biasing the arms outwardly (perhaps to zero), ensuring that the arms can be retracted and the tool removed from the borehole, even in the event of a failure of the restraining means.
Accordingly, it will be understood that for more sophisticated data-logging applications, the borehole wall-engaging contacts are required to carry sensors, for example sensors responsive to electrical resistance. With such applications, the arms are typically expanded so that the sensors engage the borehole wall adjacent the distal end of the zone of interest within the borehole (which might be the bottom of the borehole, for example), and the tool is withdrawn from the borehole with the sensors remaining in contact with the wall, continuous or discrete measurements being taken as the tool is withdrawn. The tool is typically withdrawn from the borehole by a cable connected to a winch above ground. A smooth tool motion is desirable so that measurements can be taken at all required positions within the zone of interest, i.e. it is desired to avoid the tool becoming stuck. If the tool becomes stuck, even momentarily, the cable will extend resiliently until the tension therein overcomes the friction restraining the tool, whereupon the tool will move rapidly, removing some or all of the extension from the cable. During this rapid movement rock strata might be passed without suitable measurement. It is known to fit the tool with accelerometers so that the evidence of sticking can be obtained, but this does not allow the missed or unsuitable measurements to be recovered. To enable the tool to move smoothly along the borehole with the sensors in contact with the wall thereof, adjustment of the contact pressure is desirable, and the drive means described can provide this. The apparatus can therefore allow optimum contact to provide suitable data-logging whilst reducing friction and component wear.
For more sophisticated applications, the arm or arms can (each) carry a sensor pad, in which case means may be provided to allow the one or more sensor pads to maintain its orientation relative to the mandrel.
The invention also provides a method of actuating the arms of a data logging tool in which the arms are retracted and restrained in their retracted position during introduction of the tool into a borehole. The drive means is actuated to load the resilient biasing means, and when the tool is in its desired position, the restraining means is released to allow the resilient biasing means to urge the arms against the wall of the borehole.